Re: lingering questions in my mind
If you read a lot of the stories in here, you'll find that a lot of your suspicions have merit. WS's rarely tell the whole truth. It's unusual for a woman to participate in an affair without deeper feelings for the AP. It's unusual for condoms to be used. "Four months" can mean "several years". Very telling that her stories and explanations have changed over time. Your need for more information is very common in BH's and very valid.
Definitely keep divorce on the table and don't let her get complacent. If you have children together, get them DNA tested to verify that they're really yours. Let her know you are doing this so that she understands the full ramifications of broken trust.
What work has she done to figure out why she did this in the first place? What work has she done to guarantee that this will never happen again? Because if the underlying causes have not been addressed, there is no way you should trust her again. As you know, your MC was wrong to suggest that you're at fault for the affair. There are a thousand ways to address problems in a marriage. Going out and screwing another man is neither a legitimate nor productive one. Your wife needs IC, not MC, to figure out what is broken in her.